1. Field of the Invention
The present invention relates to a method and apparatus for expanding an operating range of a centrifugal compressor which is, for example, used as an air feeder to a turbocharger for supercharging an engine or used as an air supply in an ordinary manufacturing plant or used together with a gas turbine.
2. Discussion of the Background
Conventionally, a turbocharger for supercharging an engine may comprise a turbine with a vane wheel, a centrifugal compressor with an impeller and a bearing casing which integrally connects the turbine to the centrifugal compressor. The vane wheel is connected to the impeller through a shaft rotatably supported in the bearing casing and is rotated by exhaust gases from the engine to rotate the impeller via the shaft. Thus, intake air is compressed by the centrifugal compressor and supplied to the engine.
A centrifugal compressor for use with a turbocharger of the type described above has a characteristic such that, as shown in FIG. 1, compressor""s characteristic curves I may exceed a surge line S into a surging region X of lower flow rate. Therefore, if the surge line S can be successfully shifted to position Sxe2x80x2 where the flow rate is lower, the centrifugal compressor can be applied in a wider or expanded operating range to the engine.
A conventional proposal in this connection is disclosed for example in JP-A-05-060097 (Japanese Patent No. 3038398). It is directed to, as shown in FIGS. 2A and 2B, a centrifugal compressor of the type wherein a housing 6 has a shroud wall 5 to provide a scrolled compression duct 3 on an outer periphery of an impeller 1 via a diffuser 2, the shroud wall 5 extending ahead of the diffuser 2 to provide an air inlet 4, and wherein a vane wheel (not shown) of a turbine is connected via a shaft to the impeller 1 and is rotated by exhaust gases from an engine to rotate the impeller 1 via the shaft, whereby intake air is compressed and supplied to the engine. The apparatus comprises: a throttle portion 7 on the shroud wall 5 adjacent to the air inlet 4 and convergent toward the impeller 1 such that air a is throttled by the throttle portion 7 and sucked through the impeller 1; an annular treatment cavity 8 in the shroud wall 5; and circumferentially extending, first and second slots or openings 9 and 10 on the shroud wall 5, the first opening 9 providing communication between the treatment cavity 8 and an impeller-side portion of the air inlet 4 or portion of the air inlet 4 adjacent to the impeller 1, the second opening 10 providing communication between the treatment cavity 8 and a portion of the air inlet 4 located somewhat ahead of the impeller-side portion of the air inlet 4, i.e., somewhat behind an end of the throttle portion 7. Thus, the first opening 9, treatment cavity 8 and second opening 10 provide a mechanism for expanding the operating range by which, during a low-flow-rate operation, part of the air a sucked by the impeller 1 is circulated to attain reduction of the flow rate in terms of the surge line.
In operation of the conventional centrifugal compressor, the air a is sucked through the air inlet 4 by rotation of the impeller 1 into a suction zone of the impeller 1 and is supplied through the compression duct 3 to a target zone. During a low-flow-rate operation, the air a which has flowed into the impeller 1 is increased in pressure due to the action of the impeller 1 so as to have high pressure in comparison with the air inlet 4 and treatment cavity 8, so that part of the air a having passed through blades of the impeller 1 can be fed through the first opening 9 and treatment cavity 8 and discharged through the second opening 10 back to the impeller 1. In this manner, air flow circulation can be attained through the use of static pressure.
As described above, in the prior structure, part of the air a sucked by the impeller 1 can be circulated so that entering into the surging region can be successfully avoided even under the operating condition where the flow rate is so low as to reach the surging region. In other words, the surge line S shown in FIG. 1 can be shifted into the position Sxe2x80x2 where the flow rate is low.
In the above-described centrifugal compressor, however, the air a circulated via the first opening 9, treatment cavity 8 and second opening 10 into the impeller 11 has flow direction as shown in FIG. 2B aligned with the rotative direction (as indicated by arrow r) of the impeller 1 or flows in so-called forward direction to the rotation of the impeller 1. Thus, as compared with a case where the air a is not circulated, expansion of the operating range may be indeed achieved. However, turning of the flow angle between before and after the impeller 1 is so small that inconveniently decreased is the Euler head which is pressure ratio between entry and exit sides of the impeller 1.
An object of the present invention is, therefore, to achieve expansion of an operating range of a centrifugal compressor of the type as described above with no decrease in the Euler head.
The invention was made to solve the above problem.
According to one aspect of the invention, there is provided a method for expanding an operating range of a centrifugal compressor, the centrifugal compressor including a shroud wall extending ahead of an outer periphery of an impeller to provide an air inlet, an annular treatment cavity in the shroud wall and first and second openings on the shroud wall, the first opening providing communication between the treatment cavity and an impeller-side portion of the air inlet, the second opening providing communication between the treatment cavity and a portion of the air inlet located somewhat ahead of the impeller-side portion of the air inlet, wherein, during a low-flow-rate operation, part of the air sucked through the impeller is fed through the first opening into the treatment cavity and is discharged through the second opening so as to be circulated. The method comprises discharging the air, which has flowed through the first opening into the treatment cavity, through the second opening as flow having a direction within a range from a direction with no whirling component to a whirling direction reverse to or conflicting with the rotative direction of the impeller. According to another aspect of the invention, there is provided an apparatus for expanding an operating range of a centrifugal compressor, the centrifugal compressor including a shroud wall extending ahead of an outer periphery of an impeller to provide an air inlet, an annular treatment cavity in the shroud wall and first and second openings on the shroud wall, the first opening providing communication between the treatment cavity and an impeller-side portion of the air inlet, the second opening providing communication between the treatment cavity and a portion of the air inlet located somewhat ahead of the impeller-side portion of the air inlet, wherein, during a low-flowrate operation, part of the air sucked through the impeller is fed through the first opening to the treatment cavity and is discharged through the second opening so as to be circulated. The apparatus comprises a number of louvers arranged in the second opening of the shroud wall, angular arrangement of the louvers being within a range from a radial arrangement to an arrangement inclined reversely to the rotative direction of the impeller.
During the low-flow-rate operation, the air fed through the first opening into the treatment cavity is, when passed through the second opening, guided by the louvers so that it is discharged as flow having a direction within a range from a direction with no whirling component to a whirling direction reverse to the rotative direction of the impeller. This prevents a decrease in the Euler head.
Instead of the louvers in the second opening, a number of guide plates may be arranged in the treatment cavity, an angular arrangement of the guide plates being within a range from radial arrangement to arrangement inclined reversely to the rotative direction of the impeller. In this structure, the flow in the treatment cavity is restricted by the guide plates each having a larger area than the louver, thereby providing directional flow guide action within a range from a direction with no whirling component to a whirling direction reverse to the rotative direction of the impeller. As a result, discharged through the second opening is the air with strong directivity not aligned with the rotative direction of the impeller.
Alternatively, guide plates may be arranged in the treatment cavity as if to be extended from their corresponding louvers. This allows the air sucked into the treatment cavity to be subjected to the directional flow guide action exerted by the guide plates and by the louvers. As a result, the air is discharged through the second opening with strong directivity not aligned with the rotative direction of the impeller.